The test information curve of the HADS based on the bifactor analysis for the general distress factor.

<p>X-axis represents severity of the general factor (theta), which had been standardized (0 being average, 1 being a standard deviation). The Y-axis represents the test information value. Test information is a kind of reliability criterion in IRT models, the bigger the test information value, the less measurement error, and better reliability. In contrast to models built using CTT, in IRT models, there is a test information value corresponding to every severity point, representing the reliability at that level of severity. We get the test information curve by connecting all these values.</p

Main parameters of bifactor analysis.

<p>Loading 1^a: factor loading on general factor.</p><p>Loading 2^b: factor loading on anxiety specific factor.</p><p>Loading 3^c: factor loading on depression specific factor.</p><p>Slope 1^d: item slop of general factor, a kind of discrimination parameter for general factor.</p><p>Slope 2^e: slopes of anxiety specific factor, a kind of discrimination parameter for anxiety specific factor.</p><p>Slope 3^f: slopes of depression specific factor, a kind of discrimination parameter for depression specific factor.</p><p>Severity 1^g: boundary severity of general factor from score 0 to 1.</p><p>Severity 2^h: boundary severity of general factor from score 1 to 2.</p><p>Severity 3ⁱ: boundary severity of general factor from score 2 to 3.</p

Distributions of anxiety and depression disorders in different diseases.

*<p><i>p</i><0.05.</p

Correlations between items and HADS scales.

<p>In anxiety and depression subscales, the highest value for each item is shown in bold type.</p><p>Definition of each item is the same with Herrero <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0047577#pone.0047577-Herrero1" target="_blank">[14]</a>.</p>**<p>All correlations are significant at the 0.01 level or above.</p